Generally, a construction machine includes hydraulic actuators such as hydraulic cylinders that drive a front work device mounted on the construction machine, operation devices operated by an operator, a hydraulic pump, and a control valve that drives internal directional control valves by operation pilot pressures in response to operation amounts of the operation devices and that controls a flow rate and a direction of a hydraulic fluid supplied from the hydraulic pump to each hydraulic actuator.
In addition, the control valve is provided with a relief valve that prevents breakage of hydraulic devices. When the construction machine conducts work such as excavation, a load pressure in response to an excavation reaction force (excavation load) is generated within each of the hydraulic actuators that drive the front work device. The relief valve opens to relieve the hydraulic fluid to a tank when an internal pressure of a hydraulic circuit reaches a predetermined set pressure in such a manner that the internal pressure does not exceed withstanding pressures of the hydraulic devices due to an increase in the load pressure. Energy of the hydraulic fluid relieved from the relief valve is released as heat and, therefore, causes a loss. To address this problem, an ordinary control valve is configured such that directional control valves for different hydraulic actuators are disposed in the same pump line in parallel and a hydraulic fluid is delivered to the actuator at the relatively low load pressure (perform the so-called diversion of the hydraulic fluid) when the internal pressure of the hydraulic circuit increases. It is thereby possible to avoid the loss caused by a relief motion while suppressing an increase in the internal pressure of the hydraulic circuit.
There is known a locus controller for such a construction machine for allowing a tip end of a front work device to converge into a target locus via a satisfactory path that always matches human feeling, irrespective of the operation amount by an operator. (refer to, for example, Patent Document 1). This locus controller computes a position and a posture of the front work device on the basis of signals from angle sensors, and computes a target speed vector of the front work device on the basis of signals from operation lever devices. The locus controller corrects the target speed vector in such a manner that the target speed vector turns toward a point forward in an excavation travel direction by a predetermined distance from a point on the target locus at the shortest distance from the tip end of the front work device, and computes target pilot pressures for driving hydraulic control valves in such a manner that target pilot pressures correspond to the corrected target speed vector. The locus controller controls proportional solenoid valves provided in an operation hydraulic circuit to generate the computed target pilot pressures.
There is also known a controller for a hydraulic construction machine that aims to improve a degree of freedom for matching among actuators that are operated by combined operation and to improve operability of the hydraulic construction machine, and that individually controls opening degrees of a plurality of control valves that control a flow of a hydraulic fluid to one of the actuators (refer to, for example, Patent Document 2). Proportional valves for generating pilot signals are attached to first and second boom control valves that control a flow of a hydraulic fluid to a boom cylinder and to first and second arm control valves that control a flow of a hydraulic fluid to an arm cylinder. This controller determines control signals in response to a boom lever stroke signal and an arm lever stroke signal by using a map set for every work mode, and controls the proportional valves by these control signals.